Composite panels

ABSTRACT

A composite panel consisting of outer skins and an inner core consisting of a foamed polymer, wherein the structure or properties of the inner core are anisotropic. The composite panel can be made by applying external heat and pressure to melt a skin of thermoplastic composite and an initial thickness of a thermoplastic core which has anisotropic properties causing the skin and core to fuse together followed by cooling the fused structure. The composite panel can be made by applying external heat and pressure to melt layers of a thermoplastic adhesive positioned between the outer skins and an inner core consisting of a foamed polymer, wherein the structure or properties of the inner core are anisotropic, so that the skins are bonded to the core by the melted layers of the thermoplastic adhesive followed by cooling the bonded structure.

This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 60/798,628 filed May 8, 2006.

BACKGROUND OF THE INVENTION

The instant application relates to sandwich panels articles and methods for their manufacture and more specifically, the instant application relates to sandwich panels with low density cores and methods for their manufacture.

Sandwich panels are well known and widely used in many applications for the combination of mechanical properties and low weight. For large structures such as buildings, boats, containers, truck boxes and the like, sandwich panels consisting of surface skins of relatively thin rigid material sandwiching a low density core are often used. These sandwich panels are particularly advantageous because the skin core structure is extremely efficient at resisting loads, especially bending loads. The reason for this is that when a bending load is applied to a structure the outer portions of the structure develop high tensile and compressive stresses while towards the center of the thickness of the panel the tensile and compressive loads become progressively less and at the center of the panel (neutral axis) there are no tensile or compressive loads. Therefore it makes sense to have a structure in which the outer layers are made from strong, stiff material while the center is made from a material with lower properties which can be cheaper and lighter. Numerous techniques exist for producing such panels with either foam, honeycomb or balsawood core and composite, wood and metal skins.

Balsa provides excellent compressive strength but is also expensive, has environmental concerns and is prone to rotting if it becomes wet either as a result of the core being breached or from moisture seeping through the edges of the panel. Many existing products use metal, NOMEX, paper or thermoplastic honeycombs as the core. Honeycomb is more expensive than foam and has very limited surface area for bonding to the skin. This can be problematic for aesthetics because of the read-through of the honeycomb pattern when skins are laminated onto it because all of the lamination pressure acts on the edges of the honeycomb cells. In addition, the bond between the edges of the honeycomb and the skin needs to be relatively strong in order to prevent delamination because of the limited bond area. An alternative to a honeycomb is to use foamed polymer. Foamed polyurethane, polyisocyanurate, polystyrene, polyethylene, polypropylene, SAN, PVC and other foamed materials are widely used. Foamed polymer is often cheaper because it can be manufactured in a continuous extrusion process while honeycombs typically involve making sheet which is then slit, deformed and welded into a hexagonal structure. Foamed polymer also eliminates the problems of read-through of the honeycomb structure and offers greater surface area for bonding. However, foamed polymer core composite panels typically have lower physical properties than honeycomb core sandwich panels of the same weight. Thus, it would be advantageous to have a sandwich panel made using a core material that combined the advantages of both the honeycomb and the polymer foam.

SUMMARY OF THE INVENTION

The instant invention is a sandwich panel having a core that combines the advantages of both the honeycomb and the polymer foam. More specifically, the instant invention is a sandwich panel, comprising: outer skins of a rigid material and an inner core comprising a foamed polymer, wherein the cells of the foamed polymer are anisotropic in structure.

The instant invention is also a process for the manufacture of such a sandwich panel comprising the step of bonding the skins to the core. Alternatively, the instant invention is also a process for the manufacture of a composite panel comprising the steps of: (a) applying external heat and pressure to melt layers of a thermoplastic adhesive positioned between the outer skins of a composite material comprising fibers in a polymer matrix and an inner core comprising the foamed polymer so that the skins are bonded to the core by the melted layers of the thermoplastic adhesive; and (b) cooling the bonded structure.

The instant invention is also a truck box, a deck plank, a scaffolding plank, a table, an article of furniture, a desktop, a concrete formwork panel, a boat hull, a blast panel a building structure, a sunroof, a headliner, a door panel, a parcel shelf, a load floor, a visor, a seat component, or an automobile part comprising the sandwich panel of the instant invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a corner of a sandwich panel of the instant invention showing outer skins of a rigid material and an inner core comprising a foamed polymer, wherein the structure and properties of the inner core are anisotropic;

FIG. 2 a is a perspective view of a corner of a foamed polymer core for use in the instant invention having elongated bubbles therein.

FIG. 2 b is a perspective idealized view of a corner of a foamed polymer core for use in the instant invention having bubbles therein of smaller size toward the faces of the core;

FIG. 3 a is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising geometric tube shaped structures therein oriented perpendicularly to the faces of the panel;

FIG. 3 b is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising geometric bar shaped structures therein oriented perpendicularly to the faces of the panel.

FIG. 3 c is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising geometric rod shaped structures therein oriented perpendicularly to the faces of the panel.

FIG. 4 a is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising a geometric honey comb shaped structure therein oriented perpendicularly to the faces of the panel;

FIG. 4 b is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising a geometric square shaped structure therein oriented perpendicularly to the faces of the panel;

FIG. 4 c is a perspective view of a corner of a foamed polymer core for use in the instant invention further comprising a geometric triangle shaped structure therein oriented perpendicularly to the faces of the panel;

FIG. 5 is a perspective view of a corner of a sandwich panel of the instant invention having a multi-layer construction;

FIG. 6 is an end perspective view of a sandwich panel of the instant invention overcoated with a polymer layer; and

FIG. 7 is an end view of a sandwich panel of the instant invention overcoated with a polymer layer which incorporates edge and face features.

DETAILED DESCRIPTION

Referring now to FIG. 1, therein is shown a sandwich panel 10 of the instant invention comprising a foamed polymer core material 13 whose thickness (‘z’ direction) is small in comparison to its length and width (‘x’ & ‘y’ directions). The sandwich panel 10 also comprises composite skins 11 and 12 of a rigid material (i.e., more rigid than the foamed polymer core material 13) bonded to the faces of the foamed polymer core material 13. The matrix of the composite skins and the material of the core may be selected to be compatible thermoplastics, meaning that when brought together under heat and pressure they will bond to each other. Alternatively the core material and the composite matrix of the skin material may be incompatible materials which are bonded by the addition of an intermediate layer of thermoplastic or adhesive between the two. The core material has a structure and properties which are inhomogeneous and/or an-isotropic.

Referring now to FIG. 2 a the core material 14 is a continuous sheet of foamed polymer in which the bubbles or cells 15 thereof are elongated and tubular or ovaloid in shape with the longest axis in the ‘z’ direction. The core material 14 has increased compressive strength compared with the more usual spherical bubble foam. The core material 14 is commercially available as IMPAXX brand foamed polystyrene from The Dow Chemical Company.

Preferably the polymer of the foamed polymer core of the instant invention is a thermoplastic polymer. More preferably the polymer of the foamed polymer core of the instant invention is a thermoplastic selected from general purpose polystyrene (GPPS), high impact polystyrene (HIPS), SAN, ABS, polypropylene (PP), polyethylene (PE), PET, and PVC. Preferably the foamed polymer core of the instant invention has an average density between 0.5 pound per cubic foot (pcf) and 20 pcf, more preferably between 1 and 8 pcf, and most preferably between 2 pcf and 6 pcf.

Referring now to FIG. 2 b, therein is shown a second embodiment of a core of foamed polymer 16 in which the bubbles which make up the foam are essentially round and vary in size, wherein the bubbles 17 which are close to the center of the core 16 are larger than the bubbles 19 which are closer to the surface of the core 16 (preferably the bubbles are progressively smaller toward the surface of the core 16). As a result the material at the center has a lower density while the material at the surface has a higher density. The higher density of the material at the surface provides greater strength, particularly compressive strength and also provides greater area to bond to the skins.

Referring now to FIGS. 3 a, 3 b and 3 c, therein is shown a third embodiment of the instant invention wherein the core material 20, 22 and 24, respectively, consists of a foamed polymer which contains discrete strands, planes or geometric shapes of more dense or solid thermoplastic material 21, 23 and 25, respectively, which are continuous in the ‘z’ direction as shown. Such geometric shapes may be, for example, discrete rods, and strips or round, square or hexagonal tubes. These more dense or solid regions increase the compressive properties of the foam. The increase in strength and stiffness is out of proportion to the increase in weight because although the solid material of these pillars is thin, it is supported by the foam around it so that it is less likely to buckle when subjected to compressive loads. The more dense or solid material may be the same material (preferably a thermoplastic polymer) as the material of the foam or may be a different but compatible material. The percentage of more dense or solid sections may vary but preferably the percentage is small, in the region of 0.1% to 5% of the total cross section.

Referring now to FIGS. 4 a, 4 b and 4 c, therein is shown a fourth embodiment of the instant invention wherein the geometric shapes of the more solid or dense material in the Z direction 27, 29 and 31, respectively, are connected to form a continuous array in the foamed polymer 26, 28 and 30, respectively. Such connectivity further enhances the compressive strength of the foam as each element may now receive support from adjacent elements when overloaded. The connectivity also increases the shear strength and stiffness of the core structure. Such materials are commercially available from The Dow Chemical Company under the trade name “STRANDFOAM”.

In a fifth embodiment of the instant invention the areas of continuous material in the ‘z’ direction are made up of a composite consisting of either discontinuous or continuous fibers in a matrix of a thermoplastic material. Again these areas can disproportionately increase the compressive properties of the core because the thin column-like elements which would otherwise be prone to buckling are supported by surrounding foam.

In a sixth embodiment of the instant invention the density of the core may be varied in the ‘Z’ direction. Preferably the density variation would be such that the lowest density region is in the center of the core, the highest density region at the surface of the core where the skins are bonded and would vary gradually between the lowest and highest density regions. This variation in density may be achieved while the foam is being manufactured, in an intermediate step after the manufacture of the foam before the skins are bonded to the panels or preferably during the process of bonding the skins to the core. The latter may be achieved for example by the use of temperature and pressure during the lamination of the skins to the core. As the skin and outer portion of the core heat up the outer portion of the core softens and compacts under pressure. The temperature gradient results in greater softening of the foam closer to the surface and therefore greater consolidation in the outer portion of the core with progressively less toward the center of the core and at a certain distance from the surface, no further consolidation, resulting in a density gradient within the core and similarly graduating properties. These varying properties within the core enhance the performance of the panel.

It should be understood that while for most structural applications it is preferred that the anisotropy results in higher properties of the foam in the Z direction there are also instances where it is advantageous that the anisotropy results in lower mechanical properties in the Z direction. An example of this would be where gas barrier properties are important in the panel. In this case the denser array described in the fourth embodiment may be advantageously arranged so that the axis of the repeating shapes lies in the x or y direction as this will result in multiple walls in the plane of the panel and contribute to the gas barrier properties of the foam.

In all cases, for economic reasons, the preferred method of manufacture of the foam core is by extrusion and foaming of a single polymer or by co-extrusion of two or more polymers with at least one of them being foamed. If the foamed and solid regions of the core are of the same polymer it is possible to produce both with a single die and single extruder by appropriate use of openings in the die to encourage foaming in some areas while discouraging foaming in others. These techniques are well known in the art. The core and skins of the panel may be joined by the application of heat and pressure. The heat may be applied by contacting the outer portion of the skin with a hot surface and causing the heat to permeate through the skin such that the hot skin contacts the foam and causes it to heat up. In this way the outer portions of the core are heated to the point of melting/fusing to the skin while the center of the core remains cooler.

The machine for applying the heat and pressure is preferably one which is capable of applying heat and pressure for consolidation and also of cooling. This can be achieved with a belt press or flat bed laminator with a first heating region, followed by a second cooling region. It may also be achieved using two linked reciprocating presses, the first press having heated platens, the second having cooled platens. The presses each open and close on a belt mechanism. The panel components are assembled onto the belt prior to the heated press. As the heated press opens the belt carries the components into it. The heated press closes and heats and applies pressure to the panel assembly causing the rigid skins and the outer portions of the core to heat and fuse together. The heated press opens and the belt carries the hot panel into the cooling press. The cooling press closes and cools and consolidates the panel. The cooling press then opens and the belt carries the cooled panel out to an unloading station.

It will be understood that as multiple panels are produced all stations will be operating simultaneously, that is, as one panel is being assembled the previously assembled panel is in the heated press, while the previously heated panel is in the cooling press and the previously cooled panel is at the unloading station. In this way the productivity of the equipment is maximized. Many forms of belt may be suited to carry out the transport function but one form is a silicone or PTFE coated glass cloth. The belt is on a roll prior to the assembly station, extends through the press and to a second roll after the unloading station. The second roll is driven to pull the belt from the first roll. As the presses close the belt may be stopped either by interlinked controls between the presses and the roll or by employing a clutch on the roll drive which will stop it rotating as the presses close causing a drag on the belt. Such equipment and controls are widely available and understood in the extrusion and weaving industries. Once all the belt has been transferred from the first to the second roll the process would be briefly stopped while the belt is re-wound onto the first roll.

Alternatively, such may also be achieved by the use of pairs of heated and cooled rolls where the composite skins and core are fed into pairs of rotating heated rolls which heat the composite and melt the surface of the core, followed by cooled rolls which consolidate the skins onto the core as the materials at the interface cool and solidify. Or, such may also be achieved by a pultrusion process where continuous sheets of skin material are pulled through a die with discontinuous sheets of the anisotropic foam core fed between. The entrance portion of the die is used to apply heat and pressure to the thermoplastic composite skins, causing them to be compressed against the foam core and heat it so that the skin and core melt fuse to each other. The exit portion of the die is used to cool and consolidate the panel.

The skin may consist of a single layer of sheet or multiple layers of sheet which are consolidated to each other and to the core during the process. Referring now to FIG. 5, other layers may be included, either for improved properties of the panel 32 or to ease the manufacturing process. An additional layer 35 may be included between multiple layers of composite 34 and 36. These can be used to increase the toughness of the panel as these non-composite layers can be more ductile and help to spread extreme impacts across a wider area of the panel. The layer 35 could be a compatibilizing layer, designed to achieve a bond between two different skin materials 34 and 36 which are not sufficiently compatible to be bonded by heat and pressure alone. An additional layer 37 may be included between the inside of the skin 36 and the outside of the core 38. This layer 37 may be an adhesive or compatibilizer layer designed to achieve good bond between the skin 36 and core material 38 which would not otherwise bond using heat and pressure alone. If desired, a decorative layer 33 can be bonded to the skin 34.

The compatibilizer layers used between different layers may be thermoplastic films designed to bond to different substrates. These may be monolithic films of materials such as EVA or modified EVA which are widely used as bonding layers between many kinds of materials or the bonding layer may itself consist of two or more layers. For example a two layer structure consisting of EVA and polypropylene. The EVA side creates a bond to a polystyrene core while the polypropylene side creates a bond to a polypropylene matrix composite skin. Note a monolithic EVA layer would also bond to the PP matrix composite but a two layer structure with EVA extruded onto PP film PP is cheaper to manufacture since it necessary to use a backer film in extruding EVA (typically a silicone coated release paper). This release paper adds to the cost of the tie layer and adds expense to the panel manufacturing process as it must be removed and disposed of. If the EVA is extruded onto a PP film in lieu of a disposable backing paper the EVA can be heat bonded to the polystyrene core while the PP is bonded to the PP matrix composite skin.

Referring now to FIG. 6, therein is shown a sandwich panel of the instant invention consisting of a foamed polymer core 41 and skins 39 and 40 over extruded with a thermoplastic polymer material 42. The layer 42 may be used to improve the surface smoothness of the panel, improve surface properties such as scratch or u/v resistance or to provide a decorated surface by use of a colored, printed or decorated film.

Referring now to FIG. 7, therein is shown a sandwich panel of the instant invention consisting of a foamed polymer core 45 and skins 43 and 44 over extruded with a thermoplastic polymer material 46 formed to have a tongue 47 on one edge, a groove 48 in the other edge and ribs 49 on the top surface.

The rigid skins to be bonded to anisotropic foamed polymer according to the instant invention may be produced by any suitable means and may be any suitable material. Examples include: thermoplastic composites such as Fulcrum Composites Inc. (Midland, Mich.) thermoplastic pultrusion, creation of thin thermoplastic ‘pre-preg’ tapes bonded together like plywood; thermoplastic composite panels produced using consolidated co-mingled fibers, thermoplastic composite panels produced by impregnating sheets of fiber with polymer emulsions or plastisols and other means; thermoset composite materials: extruded sheets of any unfilled thermoplastic, extruded sheets of filled or reinforced thermoplastic; sheets of wood or wood based materials such as veneers, plywood, laminated veneer lumber (LVL), MDF, fiberboard, chipboard, hardboard, oriented strand board (OSB); sheets or foils of metal, sheets of paper, cardboard or other similar materials, monolithic or laminated sheets of glass and sheets of mineral or stone-like materials such as concrete, stone, mica, drywall, plasterboard, artificial stone, cultured stone or other rigid sheet materials. The sandwich panels of the instant invention can be used in an almost unlimited number of applications of which the following are only a few specific examples: truck boxs, deck planks, scaffolding planks, tables, articles of furniture, desktops, concrete formwork panels, boat hulls, blast panels, building structures, sunroofs, headlinesr, door panels, parcel shelfs, load floors, visors, seat components, and automobile parts.

While the instant invention has been described above according to its preferred embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the instant invention using the general principles disclosed herein. Further, the instant application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims. 

1. A sandwich panel, comprising: outer skins of a rigid material and an inner core comprising a foamed polymer, wherein the cells of the foamed polymer are anisotropic in structure.
 2. The sandwich panel of claim 1, wherein the outer skins comprise a rigid sheet material selected from the group consisting of a composite sheet material incorporating reinforcing fibers and a matrix of thermoset or thermoplastic resin, an extruded thermoplastic sheet, a co-extruded thermoplastic sheet, a sheet of wood, a sheet of fiberboard, a sheet of chipboard, a sheet of plywood, a sheet of laminated veneer lumber, a sheet of metal and a sheet of glass.
 3. The sandwich panel of claim 1, wherein the cells of the foamed polymer core comprise elongated or ovaloid bubbles oriented perpendicularly to the faces of the panel.
 4. The sandwich panel of claim 1, wherein the density of the foamed polymer core decreases toward the center of the core.
 5. The sandwich panel of claim 1, wherein the foamed polymer core contains geometric shapes of denser material oriented perpendicularly to the faces of the panel.
 6. The sandwich panel of claim 1 in which the core and skins are bonded together by an intermediate layer of thermoplastic material which is compatible with both the skins and the core materials.
 7. The sandwich panel of claim 1, wherein the material of the foamed polymer core is selected from the group consisting of polystyrene, high impact polystyrene, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, polyvinylchloride, polyethylene-teraphthalate, polyethylene and polypropylene.
 8. The sandwich panel of claim 1 where the core is manufactured by extrusion.
 9. The sandwich panel of claim 2 where the rigid sheet material of the outer skins is a composite which has a thermoplastic matrix and is manufactured by a process selected from the group consisting of pultrusion, the consolidation of woven or aligned thermoplastic composite tapes and the consolidation of co-mingled fibers.
 10. The sandwich panel of claim 2 where the rigid sheet material of the outer skin is a composite which has a thermoset matrix and is manufactured by a process selected from the group of pultrusion and a belt press process.
 11. The sandwich panel of claim 1 encapsulated with a layer of thermoplastic polymer.
 12. A truck box, a deck plank, a scaffolding plank, a table, an article of furniture, a desktop, a concrete formwork panel, a boat hull, a blast panel a building structure, a sunroof, a headliner, a door panel, a parcel shelf, a load floor, a visor, a seat component, or an automobile part comprising the sandwich panel of claim
 1. 13. A process for the manufacture of the sandwich panel of claim 1 comprising the step of bonding the skins to the core
 14. The process of claim 13 where the bonding is achieved using a thermoplastic adhesive layer.
 15. The process of claim 14 where the bonding is achieved using a thermoset adhesive layer.
 16. A process for the manufacture of the sandwich panel of claim 1 comprising the steps of: (a) applying external heat and pressure to melt layers of a thermoplastic adhesive positioned between the outer skins of a composite material comprising fibers in a polymer matrix and an inner core comprising the foamed polymer so that the skins are bonded to the core by the melted layers of the thermoplastic adhesive; and (b) cooling the bonded structure.
 17. The process of claim 13, further comprising the step of overextruding an outer layer of thermoplastic around the sandwich panel. 